In general, a color cathode ray tube includes an envelope having a substantially rectangular panel, a phosphor screen formed of phosphor layers, emitting three colors, formed on the inner surface of an effective region of the panel, and a substantially rectangular shadow mask disposed inside the phosphor screen in a face-to-face relationship. A multiplicity of apertures serving as electron beam passage apertures are formed in an effective surface of the shadow mask in a predetermined arrangement. Three electron beams emitted from an electron gun disposed in a neck of the envelope impinge upon predetermined phosphor layers according to selection performed by the shadow mask, thereby displaying a color image.
In such a color cathode ray tube, each of the apertures in the shadow mask is formed in a substantially rectangular configuration. A plurality of rows of apertures are arranged to extend vertically and arranged in the horizontal direction at predetermined spacing. Each of the aperture rows includes a plurality of apertures which are arranged in the vertical direction with a bridge portion being interposed between two adjacent apertures. Each of the phosphor layers in three colors on the phosphor screen is formed like elongate stripes extending vertically, relative to the shadow mask.
With the shadow mask of this type has been provided in which the width W of the apertures in the horizontal direction is gradually increased or kept constant from that of the aperture located at the center of the effective surface of the shadow mask to that of apertures located in the horizontal peripheries of the effective surface and in which the apertures in the same row of apertures extending vertically have the same width W. Such an aperture width W in the horizontal direction is expressed by: EQU D=C+K1.multidot.X.sup.2
where C represents the horizontal width of the aperture at the center of the effective surface; (X, Y) represents the coordinates of the aperture in an arbitrary position of the effective surface relative to the horizontal axis (x-axis) and the vertical axis (y-axis) which perpendicularly cross each other on the center of the effective surface; and K1 is a coefficient.
In general, phosphor layers in three colors in the form of stripes that constitute a phosphor screen are formed by a photographic printing process in which the shadow mask to be incorporated in the color cathode ray tube is used as an optical mask. Specifically, the phosphor screen is irradiated with light from a light source of an exposing device through the shadow mask to print the phosphor layers in three colors on the phosphor screen. However, when phosphor layers in three colors are printed by applying the shadow mask as described above to a flat tube wherein the effective region of the panel is nearly planar and a deflection angle is about 110.degree., a phenomenon occurs wherein phosphor layers become undesirably narrow in the vicinity of the ends of diagonal axes of the panel effective region where illuminance of the light from the exposing device is lowest. Further, depending on the relationship between the position of the light source, the curvature of the shadow mask and the curvature of the panel, a phenomenon may be encountered wherein the phosphor layers thus formed meander and a corrective action to this makes the phosphor layers narrower.
When phosphor layers in the vicinity of the end portions of the diagonal axes are narrowed as described above, brightness is lowered in the vicinity of the corners of the display screen to give dark appearance, which results in a problem in that the so-called color purity is reduced.